The invention is based on a power supply unit operating with a battery, as often used, for example, in portable appliances in consumer electronics. Appliances of this type need protection against polarity reversal of the battery voltage, since, in the case of specific battery types, batteries can be inserted the wrong way round and, as a result of an inverted battery voltage, electrical components belonging to the appliance, in particular electrolytic capacitors, can be destroyed.
To provide protection against polarity reversal of the battery, it is known to arrange a diode or a switching element in series with the battery. However, diodes have the disadvantage that they have a relatively high forward voltage and therefore exhibit not inconsiderable losses, which is undesirable specifically during battery operation. More suitable as a protection against polarity reversal is therefore a switching transistor, in particular a MOSFET, which has a relatively low forward resistance when it is switched on fully. The base of a MOSFET is in this case wired in such a way that the MOSFET is turned on completely with the correct battery polarity and blocks if the battery polarity is wrong. Such protective circuits to counter polarity reversal of the battery are disclosed, for example, by DE-A-196 03 117 and EP-A-0 918 389.
The object of the invention is to specify a power supply unit operating with a battery and comprising a switching transistor which is arranged in the current path of the power supply unit, in which the switching transistor is protected against destruction.
The power supply unit of the invention, which has a switching transistor whose current terminals are arranged in series with a battery, in the current path of the power supply unit, has a protective circuit which is arranged across the current terminals of the switching transistor and evaluates the differential voltage across the switching transistor. In the event of too high a differential voltage, the power supply unit or the appliance is switched off, so that the switching transistor cannot be destroyed.
In particular if the switching transistor used is a MOSFET, which usually has a parallel-connected internal diode, there is a risk to the latter if it is not turned on at a high current or not turned on correctly. The internal diode of the MOSFET is normally by nature a poor diode with a high forward voltage. In particular in appliances with a high current consumption, this diode would heat up substantially and, in this case, constitute a safety problem. The protective circuit contains in particular a threshold circuit which reacts in the event of too high a differential voltage across the current terminals of the switching transistor, so that there is no risk to the MOSFET.
In the event of too high a differential voltage, the protective circuit can, for example, switch off the appliance per se or significant parts of the appliance, so that the current flow is reduced to a non-hazardous value. If the power supply unit has a DC-DC converter, then the switching transistor of the DC-DC converter can advantageously also be switched off directly. Battery-operated power supply units with a DC-DC converter are needed in particular for portable television sets.
The protective circuit used can in particular be a simple transistor stage with a transistor whose base and emitter are connected across the current terminals of the switching transistor and whose base bias voltage is set via a voltage divider in such a way that, in the event of too high a differential voltage, the transistor switches on and, in this way, triggers a turn-off signal via the collector.